"picture_url" represents a website location where the pic is stored. You can not add a picture from a file on your computer. There are many websites that hold pics for free. Photobucket is the easiest and most common. Make an account (I've got 3), load up pic from computer to site, place in folder if you like, photobucket gives you the url for forums, copy/paste it. Simple.
for pics in the forum, right click, scroll down to properties, that gives you the website location of the pic.